docs: def periodic_distance(a, b, periodic):
      '''
      Periodic distance between two arrays. Periodic is a 3
      dimensional array containing the 3 box sizes.
  
      '''
      a = np.array(a)
      b = np.array(b)
      periodic = np.array(periodic)
  
      delta = np.abs(a - b)
      delta = np.where(delta > 0.5 * periodic, periodic - delta, delta)
      return np.sqrt((delta ** 2).sum(axis=-1))